Tommy C. Hopkins

Constant-Rate-of-Strain and Controlled-Gradient Consolidation Testing

Controlled-gradient (CG), constant-rate-of-strain (CRS), and conventional incremental-loading (STD) consolidation testing are compared and evaluated. Undisturbed samples of three soils common to Kentucky were used in the testing program. Results of 15 CG, 14 CRS, and 32 STD consolidation tests are evaluated. The feasibility of these new test methods for routine testing is briefly discussed and recommendations are made for refinements in testing procedures.

Identification of Shales

Engineering tests were performed on 40 different types of shales. Both hard and soft shales, as well as shales having histories of embankment failures and shales having little known involvements, were tested. The suitability of ten different slake-durability test procedures was evaluated as a means of broadly characterizing the engineering performance of Kentucky shales. Two procedures devised during the study appeared to better characterize slake-durability properties than procedures previously proposed. Natural water contents and jar slake tests were performed to determine if such tests might provide a fairly rapid means of identifying troublesome shales. The natural water content of a shale was a strong indicator of the slake-durability properties. Swelling properties of ten shale types were examined. A good correlation was obtained between a newly devised slake-durability index and the water content of a shale after swelling was completed. When exposed to water, most of the shales exhibited high swell pressures.

Resilient Modulus of Compacted Crushed Stone Aggregate Bases

In recent years, the American Association of State Highway Transportation Officials (AASHTO) has recommended the use of resilient modulus for characterizing highway materials for pavement design. This recommendation evolved as result of a trend in pavement design of using mechanistic models. Although much progress has been made in recent years in developing mathematical, mechanistic pavement design models, results obtained from those models are only as good as the material parameters used in the models. Resilient modulus of aggregate bases is an important parameter in the mechanistic models. The main goal of this study was to establish a simple and efficient means of predicting the resilient modulus of different types of Kentucky crushed stone aggregate bases. To accomplish this purpose, resilient modulus tests were performed on several different types of aggregate bases commonly used in pavements in Kentucky. Specimens were remolded to simulate compaction conditions typically encountered in the field. Tests were performed on wet and dry specimens. The compacted specimens were 6 in. in diameter and 12 in. in height Crushed limestone base materials included Dense Graded Aggregate (DGA), and Crushed Stone Base (CSB). Number 57s, crushed river gravel, recycled concrete, and asphalt drainage blanket samples were submitted for testing by engineers of the Kentucky Transportation Cabinet. A new mathematical resilient modulus model, developed in a previous study by researchers at the University of Kentucky Transportation Center (UKTC), was used to relate resilient modulus to any selected, or calculated, principal stresses in the aggregate base. This model improves the means of obtaining best data “fits” between resilient modulus and stresses. Furthermore, the resilient modulus can be predicted, using the UKTC resilient modulus model, when the stress condition and type of Kentucky base aggregate are known. Multiple regression analysis is used to obtain model coefficients, k1, k2, and k3, of the relationships between resilient modulus and confining and deviator stresses used in the testing procedure. Also, multiple regression analysis was performed using other models developed by the National Cooperative Highway Research Program (NCHRP Project 1-37A, 2001) and Uzan (1985) to obtain the model coefficient, k1, k2, and k3. The resilient modulus data and the UKTC model, as well as models developed by NCHRP and Uzan, are readily available to design personnel of the Kentucky Transportation Cabinet. Computer software was developed in a client/server and Windows environment. This program is embedded in the Kentucky Geotechnical Database, which resides on a Cabinet server in Frankfort, Kentucky.

Modification of Highway Soil Subgrades

Major study objectives were to develop highway pavement subgrade stabilization guidelines, to examine long-term benefits of chemical stabilizers, such as cement, hydrated lime, and two byproducts from industrial processes, and to establish a subgrade stabilization program in Kentucky. In developing a program, a number of design and construction issues had to be resolved. Factors affecting subgrade behavior are examined. Changes in moisture content and California bearing ratio (CBR) strengths of untreated and chemically treated subgrades at three experimental highway routes were monitored over a 7-year period. CBR strengths of the untreated subgrades decreased dramatically while moisture contents increased. CBR strengths of subgrade sections treated with hydrated lime, cement, and multicone kiln dust generally exceeded 12 and increased over the study period. At four other highway routes ranging in ages from 10 to 30 years, CBR strengths of soil-cement subgrades exceeded 90. Knowing when subgrade stabilization is needed is critical to the development of an economical design and to insure the efficient construction of pavements. Bearing capacity analyses using a newly developed, stability model based on limit equilibrium and assuming a tire contact stress of 552 kPa show that stabilization should be considered when the CBR strength is less than 6.5. For other tire contact stresses, relationships corresponding to factors of safety of 1 and 1.5 are presented. Stability analyses of the first lifts of the paving materials show that CBR strengths of the untreated subgrade should be about 9 or greater. Guidelines for using geogrids as subgrade reinforcement are presented. Factors of safety of geogrid reinforced granular bases are approximately 10 to 25% larger than granular bases without reinforcement. As shown by strength tests and stability analysis, when the percent finer than the 0.002 mm-particle size of a soil increases to a value greater than about 15%, the factor of safety decreases significantly. Guidelines are also presented for the selection of the design strengths of untreated and treated subgrades with hydrated lime and cement. Based on a number of stabilization projects, recommended design undrained shear strengths of hydrated lime- and cement-treated subgrades are about 300 and 690 kPa, respectively. A laboratory testing procedure for determining the optimum percentage of chemical admixture is described. Correlations of Dynamic Cone Penetrometer and the Clegg Impact Hammer values and in situ CBR strengths and unconfined compressive strengths are presented.

Long term movements of highway bridge approach embankments and pavements

Several findings and recommendations contained in the final report as well as prior interim reports of this study have been incorporated into design practice and are contained in the Geotechnical Manual of the Division of Materials, Kentucky Department of Highways. Subsurface exploration and settlement and stability analyses are now routinely performed at bridge approach sites where the embankments are twenty (20) feet or more in height or where poor foundation conditions exist. This policy is a direct result of information gained during the course of the study. Hence, site specific investigations are conducted. As a result of this study, the long-term target factor of safety is 1.8 (geotechnical manual). When 1.8 cannot be obtained using available fill material, select granular material (Standard Drawings RGX 100 and RGX 105) is recommended. However, regardless of the results of the stability analyses, the geotechnical manual requires that select granular material be used when there are ample quantities of suitable granular material available from roadway excavation or when there is a likelihood of undesirable materials, such as soil-like shales, being used to construct the approach embankments. In situations where granular material must be processed or transported long distances, the geotechnical manual recommends that it may be more economical to flatten embankment slopes with a longer bridge structure.

Long-Term Monitoring of Culvert Load Reduction Using an Imperfect Ditch Backfilled with Geofoam

Rigid culverts resting on unyielding foundations are frequently required in Kentucky for routing streams beneath highway embankments because of shallow depths to bedrock, rolling terrain, numerous streams, and the need to use high fills, which create large vertical stresses. Expensive structures are usually required. To find a way to reduce large vertical pressures, ultralightweight geofoam was placed in a 2-ft-thick trench above a reinforced, rigid concrete box culvert resting on an unyielding foundation at a roadway site in Kentucky. In situ measurements of stresses, strains, and geofoam settlements obtained over 5 years showed that geofoam was an ideal compressible material to use in the imperfect trench. At two culvert sections where geofoam was used, vertical pressures were reduced to about 10% of the normal pressures measured at the top of a culvert section where geofoam was not used. Stresses on top of two culvert sections where geofoam was used remained relatively stable after final grade elevation was reached. Earth pressures on the sidewalls of all three sections of the culvert did not change significantly after completion of the fill and were much lower than the vertical pressures measured in the culvert section without geofoam. Geofoam settlement of 60% has been recorded. Settlement behavior of the geofoam showed that movement of the soil prism above the imperfect trench was rapidly decreasing with increasing time and suggested that the reduced vertical stresses observed under the geofoam culvert sections could remain throughout the culvert design life.

Selection of Design Strengths of Untreated Soil Subgrades and Subgrades Treated with Cement and Hydrated Lime

Selection of design strengths of soil subgrades and subgrades treated with cement or hydrated lime is a problem in pavement design analysis and construction. Different types of soils may exist in a highway corridor and different strengths may exist after the soils are compacted to form the pavement subgrade. The selected subgrade strength will largely affect the pavement thickness obtained from the design analysis, future pavement performance, and the overall bearing capacities of the subgrade during construction and the pavement structure after construction. In developing the proposed selection scheme, a newly developed mathematical model, based on limit equilibrium, is used. Relationships among undrained shear strength [or California bearing ratio (CBR)] and tire contact stresses are developed for factors of safety 1.0 and 1.5. The minimum subgrade strength required to sustain anticipated construction tire contact stresses during construction is determined. A criterion is proposed for determining when subgrade stabilization is needed and methods of selecting the design subgrade strength are examined. A least-cost analysis appears to be an appropriate approach as shown by analysis of a case study involving pavement failures. Two case studies show that soaked laboratory strengths appear to be fairly representative of long-term field subgrade strengths. Hence, using soaked laboratory strengths and least-cost analysis appears to be a reasonable means for selecting the design strength of subgrades for pavement analysis. To avoid failures of chemically stabilized layers, relationships among thicknesses of chemically treated layers and the CBR values of the untreated subgrade for a factor of safety of 1.5 are presented.

Stress Reduction by Ultra-Lightweight Geofoam for High Fill Culvert: Numerical Analysis

The study of earth pressure distribution on buried structures has a great practical importance in constructing highway embankments above pipes and culverts. Based on Spanglers research, the supporting strength of a conduit depends primarily on three factors: 1. the inherent strength of the conduit; 2. the distribution of the vertical load and bottom reaction; and, 3. the magnitude and distribution of lateral earth pressures which act against the sides of the structure. Considering high fills above them and high earth pressures they may experience, rigid culverts are usually used underneath highway embankments. To reduce high vertical earth pressures acting on buried structure, ultra-lightweight Geofoam was placed above a culvert in the field, in Russell County, KY. Before construction began, numerical analysis using FLAC 4.00 (Fast Lagrangian Analysis of Continua) had been performed to predict stresses on the culvert. Results of the analysis show that Geofoam has a great effect in reducing vertical stresses above and below the culvert. There were areas of high stress concentrations at the top and bottom of the concrete culvert if no Geofoam was placed above the culvert. After placing Geofoam above the culvert, the concentrated stress at the top can be reduced to 28 percent of the stress without Geofoam. The high stress at the bottom of culvert can be reduced to 42 percent of the stress without Geofoam. Stresses on the two sidewalls of the culvert were observed to have no significant change in values with and without Geofoam.

THE BUMP AT THE END OF THE BRIDGE

THE GENERAL RELATIONSHIP IS SUMMARIZED BETWEEN THE APPEARANCE OF BRIDGE APPROACH SETTLEMENT AND VARIOUS CONDITIONS AT BRIDGE SITES. DATA OBTAINED FROM A SURVEY OF EXISTING BRIDGE APPROACHES CONDUCTED IN THE SUMMERS OF 1964 AND 1968 PROVIDED GENERAL INFORMATION AS TO THE PREVALENCE OF EMBANKMENT OR FOUNDATION PROBLEMS IN KENTUCKY. THE APPROACHES WERE CLASSIFIED ACCORDING TO ONE OF THE FOLLOWING SETTLEMENT CATEGORIES: (1) GROUP 1 SETTLEMENT, NO MAINTENANCE NECESSARY AND NO APPROACH FAULT NOTICABLE, (2) GROUP 2 SETTLEMENT, NO MAINTENANCE PERFORMED, HOWEVER, AN APPROACH FAULT WAS OBSERVED, AND (3) GROUP 3 SETTLEMENT, MAINTENANCE PERFORMED ON THE APPROACH. THE CRITERION USED TO DISTINQUISH BETWEEN GROUPS 1 AND 2 WAS WHETHER OR NOT A BUMP WAS EVIDENT WHEN AN AUTOMOBILE PASSED ONTO OR OFF THE BRIDGE DECK. ADDITIONAL INFORMATION WAS OBTAINED BY VISUALLY INSPECTING EACH APPROACH. THE AGES OF THE APPROACHES WERE NOTED. FROM THESE DATA, IT IS EVIDENT THAT PRESENT DESIGN AND CONSTRUCTION PROCEDURES ARE NOT SUFFICIENT TO GUARANTEE SMOOTH BRIDGE APPROACHES. A COMPARISON OF PORTLAND CEMENT CONCRETE AND BITUMINOUS CONCRETE APPROACHES SHOWS A MARKEDLY HIGHER PERCENTAGE OF BITUMINOUS CONCRETE APPROACHES WITH PATCHING THAN RIGID APPROACH PAVEMENTS WITH MUD JACKING IN 1964. HOWEVER IN 1968, THE DIFFERENCE IN PERCENTAGE OF MUD JACKED AND PATCHED APPROACHES, AS WELL AS SMOOTH APPROACHES, WAS ALMOST INSIGNIFICANT. APPARENTLY FOR A SHORT PERIOD OF TIME THE RIGIDITY OF PORTLAND CEMENT CONCRETE PAVEMENT REDUCED THE OCCURRENCE OF THE APPROACH FAULT BY BRIDGING THE PRESUMED DEPRESSMENT BEHIND THE ABUTMENT. A COMPARISON OF THE MOST COMMONLY USED TYPE OF ABUTMENTS WITH RESPECT TO THE 3 SETTLEMENT GROUPS REVEALED THAT THE OPEN-COLUMN (OPEN-END) TYPE WAS MORE COMMONLY ASSOCIATED WITH SETTLEMENT GROUP 3 THAN EITHER THE PILE-END-BENT (OPEN-END) TYPE OR STUB (CLOSED-END) TYPE IN 1964. HOWEVER, IN 1968, THERE WAS AN INCREASE IN PERCENTAGE OF FAULTED APPROACHES FOR ALL TYPES OF ABUTMENTS WITH THE PERCENTAGE FOR PILE-END-BENT INCREASING THE MOST. THERE WERE SMALL DIFFERENCES IN PERCENTAGES BETWEEN THE PILE-END-BENT AND OPEN-COLUMN ABUTMENT. THE PERFORMANCE OF BRIDGE APPROACHES WITH AND WITH-OUT SPECIAL GRANULAR BACKFILL WAS COMPARED. NO ADVANTAGE WAS SHOWN AND THE INFLUENCE OF DIFFERENT GEOLOGICAL AND SOIL CONDITIONS WAS ONLY SLIGHTLY NOTICEABLE.

REDUCTION OF STRESSES ON BURIED RIGID HIGHWAY STRUCTURES USING THE IMPERFECT DITCH METHOD AND EXPANDED POLYSTYRENE (GEOFOAM)

The study of earth pressure distribution on buried structures has a great practical importance in constructing highway embankments above pipes and culverts. Based on Spangler’s research, the supporting strength of a conduit depends primarily on three factors: 1) the inherent strength of the conduit; 2) the distribution of the vertical load and bottom reaction; and 3) the magnitude and distribution of lateral earth pressures which act against the sides of the structure. Rigid culverts are frequently used in Kentucky for routing streams beneath highway embankments because of rolling and mountainous terrain, numerous streams, shallow depths to bedrock, which creates unyielding foundations, and the necessity of using high fills which create large vertical stresses acting on culverts. As a means of exploring ways of reducing large vertical earth pressures acting on a buried structure, ultra-lightweight geofoam was placed in a trench above a reinforced rigid box culvert in Russell County, KY. This study provides strong evidences from both numerical model analysis and in-situ test data to indicate that geofoam is an ideal elasto-plastic material to reduce vertical load on top of a rigid culvert resting on a rigid foundation. The load on the top of a culvert can be reduced to 20% of traditional design load after 2-ft-thick geofoam is placed on top of it. Results from numerical model are more conservative when compared to actual test data. As much as 57% of settlement from geofoam has been recorded. Stresses on the top of a culvert where geofoam was placed have reached a relatively stable level which is expected at the yield point of the geofoam. This technology can be used in applications which require controlled pressure on rigid underground structure. Whether geogoam is used or not used, the model analysis and test data show that the earth pressure acting on the sidewall does not change significantly. Although the pressure acting on the sidewall is slightly higher when geofoam is used on top of the culvert only, the value is still below the design value used by the Kentucky Transportation Cabinet. Use of geofoam placed in an imperfect trench significantly reduces the vertical stresses acting on the top of the culvert.